The present invention relates to a method for producing beta-alumina solid electrolytes, and, more particularly, to a method for producing beta-alumina solid electrolytes without using magnesium-aluminum spinel which has been widely used as a magnesium source.
Beta-alumina is expected to be used as solid electrolytes such as partition walls between a positive electrode chamber and a negative electrode chamber of sodium-sulfur batteries because it is very high in ionic conductivity (namely, low in electrical resistance) of sodium ion.
Among the beta-alumina solid electrolytes, MgO-stabilized beta-alumina solid electrolytes have hitherto been produced by the following methods. That is, MgO, an Na compound and an xcex1-Al2O3 as starting materials are mixed at an appropriate ratio, followed by calcining the mixture to make a beta-alumina and milling it. Then, the resulting materials are subjected to granulation, then molded into a desired shape and fired to obtain a beta-alumina solid electrolyte.
Furthermore, recently, as disclosed in JP-A-7-272749, there is a method of directly producing beta-alumina solid electrolytes without carrying out the calcining step in which the starting materials are converted to beta-alumina. This method is performed by using a spinel compound comprising magnesium and aluminum (MgAl2O4, hereinafter referred to as xe2x80x9cmagnesium-aluminum spinelxe2x80x9d) as a magnesium source for MgO which is a stabilizer for beta-alumina.
The above method which omits the calcining step of starting materials is useful in that beta-alumina solid electrolytes can be more efficiently produced by shortening the production steps, but suffers from the problem that production cost increases because a step of synthesizing magnesium-aluminum spinel starting material used as a magnesium source is needed.
The object of the present invention is to provide a method for producing a beta-alumina solid electrolyte without the step of calcining the starting materials wherein the step of synthesis of magnesium-aluminum spinel starting material is not needed and the beta-alumina solid electrolyte can be produced at lower cost.
According to the present invention, there is provided a method for producing a beta-alumina solid electrolyte without carrying out calcination of starting materials which comprises granulating a slurry obtained by milling and mixing starting materials of an aluminum source, a magnesium source and a sodium source in water, thereafter molding the granulated product, and then firing the resulting molded product, characterized in that magnesium hydroxide is used as the magnesium source and citric acid is added to the slurry as a dispersing agent.
In the present invention, it is preferred to keep the molded product at a given temperature for a given time in the course of the firing step. Specifically, it is preferred to keep the molded product for 1-5 hours at a temperature in the range of 900-1100xc2x0 C. which is lower than the temperature at which xcex2- or xcex2xe2x80x3-alumina is synthesized, thereby converting to a spinel at least 10 mass % of the total amount of magnesium in the above magnesium hydroxide. As a result, an active spinel high in reactivity is produced in the course of firing, and a beta-alumina solid electrolyte of lower resistance can be obtained.
Moreover, amount of citric acid added to the slurry is preferably 0.01-1 mass % based on solid matter in the slurry, and the time of the addition of citric acid is preferably before milling and mixing for preparation of the slurry. Furthermore, it is preferred to use sodium carbonate and/or sodium hydrogencarbonate as the sodium source, and it is preferred that the maximum temperature of the firing is 1550-1650xc2x0 C.
In the present invention, the term xe2x80x9cbeta-aluminaxe2x80x9d is a general term for xcex2-Al2O3 (Na2O 11Al2O3), xcex2xe2x80x3-Al2O3 (Na2O 5Al2O3). xcex2xe2x80x2xe2x80x3-Al2O3, and the like, and particularly means those having a high content of xcex2xe2x80x3-Al2O3, namely, those having so-called xcex2xe2x80x3 ratio of 95% or higher.